基于电容电压精确计算的MMC环流抑制方法

Circulating Current Suppression Method for MMC Based on Capacitor Voltage Accurate Calculation

在MMC的阀级控制中,传统的子模块计算方法通常忽略了电容电压瞬时值的波动而直接以稳态平均值为基准来计算相应时刻的投入子模块数。这种近似的计算方法使桥臂中的实际投入子模块数与系统期望子模块数之间存在一定的偏差,从而引起桥臂电压与系统期望电压之间的偏差。此时,整个MMC系统为了实现电压与能量的平衡,便会产生桥臂环流的交流分量并通过桥臂电感补偿上述不平衡电压。桥臂环流的交流分量虽然只存在于MMC的内部并不影响MMC的外部输出特性,但其出现增加了MMC的桥臂电流有效值、开关电流应力、功率损耗以及波动,降低了MMC的性能,因此实际应用中通常需要对其进行抑制。鉴于此,在不增加硬件设备、测量环节以及附加控制器的前提下,从阀级控制系统中的子模块投入数目计算环节入手,提出了一种基于对MMC桥臂子模块电容电压精确计算的环流交流分量抑制方法。该方法工程实现简单,不依赖于任何调制策略,适用于任意相的MMC换流器,在实现环流抑制、提高MMC稳定可靠性的同时具有良好的应用经济性。基于Matlab/Simulink的模型验证了所提方法的正确性与有效性。

During the valve-level control strategy of modular multilevel converter(MMC),the instantaneous voltage fluctuation of capacitor is always ignored when calculating the number of the inserted sub-modules(SMs)in the traditional calculation method,which means,only the average capacitor voltage values during steady state are just taken into consideration.However,such an approximate calculation method causes a disparity in the number of inserted SMs between the actual and the expected,thereby resulting in a deviation between the arm voltage and the expected voltage.On this occasion,to offset the imbalance between the voltage and the energy,the MMC system automatically generates theAC component of the arm circulating current.Although theAC component part of the arm circulating current only exists inside the MMC converter and have no impact to the external output characteristics,the RMS value of the arm current,switching current stress,power loss and fluctuation will be increased,and the performance of MMC will be correspondingly reduced.Therefore,it is of necessity to suppress the AC component of the arm circulating current in the actual practice.In view of this background,start with the calculation of the input number of SMs in the valve-level control system,a circulating current AC component suppression method based on the accurate calculation of the capacitor voltage was proposed,where no additional hardware equipment,measuring link and additional controller were required.The proposed method boasts of simple implementation,no modulation strategy and excellent suitability for MMC of any phase,hence possessing good economic application while achieving the circulating current suppression and improving the reliability of MMC.The correctness and effectiveness of the proposed method were verified by simulation results based on aMatlab/Simulink model.